Streptococcus is a genus of bacteria that causes disease in humans and other animals. In humans, one of the most important streptococcal pathogens is Streptococcus pyogenes, the causative organism of strep throat, scarlet fever, and rheumatic fever. In cattle, streptococcal infections are a significant cause of disease, such as mastitis.
Mastitis affects virtually every dairy farm and has been estimated to affect 38% of all cows. The disease causes destruction of milk-synthesizing tissues which reduces milk production and alters milk composition. In severe cases, the productive performance of dairy cattle may be diminished permanently. Thus, mastitis continues to be the single greatest impediment to profitable dairy production. Losses associated with mastitis cost American dairy producers about 2 billion dollars per year and cost dairy producers worldwide an estimated 25 billion dollars per year.
Current mastitis control programs devised in the 1960's are based primarily on hygiene including teat disinfection, antibiotic therapy and culling of chronically infected cows. Acceptance and application of these measures has led to considerable progress in controlling contagious mastitis pathogens such as Streptococcus agalactiae and Staphylococcus aureus. However, postmilking teat disinfection and antibiotic dry cow therapy have been less effective against environmental mastitis pathogens. Studies have shown that as the prevalence of contagious mastitis pathogens was reduced, the proportion of intramammary infections (IMI) by environmental pathogens increased markedly.
Therefore, it is not surprising that environmental mastitis has become a major problem in many well-managed dairy farms that have successfully controlled contagious pathogens. In these herds, environmental streptococci account for a significant number of both subclinical and clinical IMI in lactating and nonlactating cows. Environmental Streptococcus species involved in bovine mastitis include Streptococcus uberis, Streptococcus dysgalactiae subsp. dysgalactiae, Streptococcus equinus (formerly referred to as Streptococcus bovis), Streptococcus equi, Streptococcus parauberis and Streptococcus canis. Among the environmental streptococci, S. uberis and S. dysgalactiae subsp. dysgalactiae appear to be the most prevalent, infecting mammary glands as favorable conditions arise.
In spite of the economic impact caused by the high prevalence of environmental streptococci in many well-managed dairy herds, virulence factors associated with pathogenesis of environmental streptococcal mastitis in dairy cows are not well understood. This constitutes a major obstacle for development of strategies to control these important mastitis pathogens. Consequently, strategies for controlling mastitis caused by environmental streptococci are poorly defined and currently inadequate.
A significant need exists for effective therapies to combat streptococcal infections, both in domestic animals and in people, and for effective modalities by which the presence of a streptococcal infection may be definitively diagnosed.
Survival of pathogenic microorganisms, such as Streptococci, has depended on the evolution of a range of strategies for evasion of host defenses. Associated with this evolution is the expression of a variety of virulence determinants that favor persistence of bacteria in the face of a massive inflammatory cell infiltration. In the case of bovine mastitis, it is hypothesized that adherence to and subsequent internalization of mastitis pathogens into mammary epithelial cells is an important early event in the establishment of new intramammary infections in lactating and nonlactating mammary glands of dairy cows. Virulence factors that favor adherence and internalization to host cells play a crucial role in the establishment, spread, and persistence of infection. During the last decade, research from our laboratory has focused extensively on development of in vivo and in vitro models to study host-pathogen interactions, and especially on identification and characterization of virulence factors associated with the pathogenesis of S. uberis mastitis.